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Optimizing Persistence and Initialization

By default, persistence and initialization in a control are handled by the DoPropExchange member function. In a typical control, this function contains calls to several PX_ functions (PX_Color, PX_Font, and so on), one for each property.

This approach has the advantage that a single DoPropExchange implementation can be used for initialization, for persistence in binary format, and for persistence in the so-called "property-bag" format used by some containers. This one function provides all information about the properties and their default values in one convenient place.

However, this generality comes at the expense of efficiency. The PX_ functions get their flexibility through multilayered implementations that are inherently less efficient than more direct, but less flexible, approaches. Furthermore, if a control passes a default value to a PX_ function, that default value must be provided every time, even in situations when the default value may not necessarily be used. If generating the default value is a nontrivial task (for example, when the value is obtained from an ambient property), then extra, unnecessary work is done in cases where the default value is not used.

You can improve your control's binary persistence performance by overriding your control's Serialize function. The default implementation of this member function makes a call to your DoPropExchange function. By overriding it, you can provide a more direct implementation for binary persistence. For example, consider this DoPropExchange function:

void CMyCtrl::DoPropExchange(CPropExchange* pPX)
{
   ExchangeVersion(pPX, MAKELONG(_wVerMinor, _wVerMajor));
   COleControl::DoPropExchange(pPX);

   PX_Bool(pPX, _T("BoolProp"), m_boolProp, TRUE);
   PX_Short(pPX, _T("ShortProp"), m_shortProp, 0);
   PX_Color(pPX, _T("ColorProp"), m_colorProp, RGB(0xFF,0x00,0x00));
   PX_String(pPX, _T("StringProp"), m_stringProp, _T(""));
}

To improve the performance of this control's binary persistence, you can override the Serialize function as follows:

void CMyCtrl::Serialize(CArchive& ar)
{
    DWORD dwVersion =
        SerializeVersion(ar, MAKELONG(_wVerMinor, _wVerMajor));
    SerializeExtent(ar);
    SerializeStockProps(ar);

    if (ar.IsLoading())
    {
        ar >> m_boolProp;
        ar >> m_shortProp;
        ar >> m_colorProp;
        ar >> m_stringProp;
    }
    else
    {
        ar << m_boolProp;
        ar << m_shortProp;
        ar << m_colorProp;
        ar << m_stringProp;
    }
}

The dwVersion local variable can be used to detect the version of the control's persistent state being loaded or saved. You can use this variable instead of calling CPropExchange::GetVersion.

To save a little space in the persistent format for a BOOL property (and to keep it compatible with the format produced by PX_Bool), you can store the property as a BYTE, as follows:

    if (ar.IsLoading())
    {
        BYTE bTmp;
        ar >> bTmp;
        m_boolProp = (BOOL)bTmp;
        .
        .
        .
    }
    else
    {
        ar << (BYTE)m_boolProp;
        .
        .
        .
    }

Note that in the load case, a temporary variable is used and then its value is assigned, rather than casting m_boolProp to a BYTE reference. The casting technique would result in only one byte of m_boolProp being modified, leaving the remaining bytes uninitialized.

For the same control, you can optimize the control's initialization by overriding COleControl::OnResetState as follows:

void CMyCtrl::OnResetState()
{
    ResetVersion(MAKELONG(_wVerMinor, _wVerMajor));
    ResetStockProps();

    m_boolProp = TRUE;
    m_shortProp = 0;
    m_colorProp = RGB(0xFF,0x00,0x00);
    m_stringProp.Empty();
}

Although Serialize and OnResetState have been overridden, the DoPropExchange function should be kept intact because it is still used for persistence in the property-bag format. It is important to maintain all three of these functions to ensure that the control manages its properties consistently, regardless of which persistence mechanism the container uses.

See Also

MFC ActiveX Controls: Optimization

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